~ National Lightning Safety Institute ~
The Science of Thunder
During the past decade there has been a considerable amount of information written about lightning and lightning safety. There has, however, been a void of K-12 educational textbooks and supplemental teaching materials on the acoustics (sound) produced by lightning, called thunder. Everyone has heard thunder and understands a little about it, but few know any specifics beyond the basics. This paper is primarily written for science students, teachers, and other interested individuals to provide a resource to enhance their knowledge about the origin of thunder.
Thunder is the audible pressure (compression) wave produced by lightning. Nearly all lightning is generated by thunderstorms. However,lightning has also been observed during snowstorms, in columns of billowing smoke from forest fires, in erupting volcanic debris clouds, near fireballs created by nuclear explosions, and on some planets and moons in our solar system. Lightning is a giant static electrical spark. Where there's lightning, there's thunder, and vice versa.
It was not until the turn of the 20th century that consensus was reached in the scientific community about the origin of thunder. Thunder is the sound generated by lightning produced by a sudden and violent expansion of super-heated air in and along the electrical discharge channel path. Thunder can be a sharp or rumbling sound. The intensity and type of sound depends upon atmospheric conditions and distance between lightning and the listener. The closer the lightning, the louder the thunder.
Early man probably considered lightning to be the ultimate weapon or a weapon of their gods. The Navajo Native Americans believed the Thunderbird, a mythical bird, flapped its wings and created the sound of thunder and the source of lightning was reflected sunlight from its eyes. It was the Norse god Thor, the Greek god Zeus, and the Roman god Jupiter who wielded the mighty bolt of lightning to keep man in his place. There continues today a phrase about lightning coming from a supernatural or divine power. The phrase goes something like: "Let lightning strike me if I'm ______." The word 'bolt', often used to describe lightning, has no meaning in meteorology and is an inappropriately used term.
Some of the earliest theories about thunder originated during the Greek and Roman Empires and from the Viking (Scandinavian) people. Beliefs about thunder included that it occurred before lightning, it was a burning wind, it was caused by the collision of clouds, the sound was produced by resonance between high and low clouds, and by high clouds descending and colliding onto low clouds. By the mid-19th century, the accepted theory was the vacuum theory, whereby lightning produced a vacuum along its path (channel), and thunder was due to the subsequent motion of air rushing into the vacuum. The second half of the 19th century saw the steam explosion theory, created when water along the lightning channel was heated and exploded by lightning's heat. Another theory was the chemical explosion theory that suggested gaseous materials were created by lightning and then exploded.
For simplicity, there are two types of lightning produced by thunderstorms: lightning that strikes the earth and lightning that does not. Flashes of lightning between a thunderstorm and earth are called cloud-to-ground (CG). Flashes of lightning within a thunderstorm are called intra-cloud (IC). There is roughly five to 10 times more IC than CG flashes.
Research during the last decade confirms the existence of sprites, elves, and blue jets that are unusual momentary flashes that take place far above thunderstorms discharging into the stratosphere. These events and terms were not included in the meteorological vocabulary until recently. Such flashes are not as bright or the same in appearance as discharges observed from thunderstorms. They are faint, extremely fleeting, display different colors, and produce no thunder because they occur in the upper regions of the atmosphere where there is little or no air. For additional information about sprites, jets, and elves go to the following sites: www.ghcc.msfc.nasa.gov/skeets.html or www.albany.edu/faculty/rgk/atm101/sprite.htm.
All thunderstorms go through stages of growth, maturity, and dissipation. The life span of a thunderstorm can be as short as 45 minutes or as long as 12 hours. Lightning is initiated by the attraction of positive and negative charges, but air (gases) in our atmosphere acts as an insulator to inhibit the flow of electricity between electrical polarities. When the electrical potential builds up to overcome resistance of the air, lightning will occur.
Nearly 70% of all lightning occurs in the tropical latitude band between 35° north and south latitude. Globally, 85% to 90% of lightning occurs over land because solar radiation heats land faster, causing convection (thunderstorms) to be taller and stronger. Some intense thunderstorms over land have been known to tower over 70,000 feet (21,000 m). There are 50-75 flashes to ground occurring every second on earth. In the U.S., there are over 125 million flashes of lightning annually; an estimated 25 million strike ground. The lightning capital in the U.S. is in Florida, centered between the cities of Tampa and Orlando.
The vertical extent of a CG lightning channel averages 3-4 miles (5-6.5 km) with a maximum height of about 6 miles (9.6 km). Most CG flashes originate in thunderstorms between 15,000-25,000 feet (4,500-7,600 m) above ground level in the mixed water and ice region. The record horizontal distance of a cloud flash is 118 miles (190 km) that occurred in the Dallas-Ft. Worth area.
Most lightning in the continental U.S. occurs in the eastern three quarters of the country. The Pacific Coast states in the U.S. have the least amount of lightning. Lightning is typically associated with the warm season, but has occurred in winter during heavy snowfalls. A man was struck by lightning during a blizzard in Minneapolis, Minnesota in March and another man in Vail, Colorado in April 1996. In February 2002, a 15-year-old boy was struck by lightning while sledding. Two men, one from Maine and the other from Chicago, were struck by lightning during a snowstorm in the winter of 2004-2005.
During a cloud-to-ground flash, the first stroke of lightning is downward from the cloud along the channel. A flash consists of one or more return strokes. A CG flash may have only one return stroke, but usually has more (two to three). They are called returned strokes because the flash originates in the cloud, not at the ground. The flash and strokes lower charge to ground. Then objects on the ground send up streamers to meet the leader coming down. The electrical discharge travels upward at one-third the speed of light (62,000 miles per second or 94,000 km/second). It is routinely followed by two to three downward return strokes to ground. This is why you see lightning flicker during a CG flash.
A record number of return strokes occurred at Cape Canaveral, Florida when 26 return flashes were recorded. Research has revealed that during a CG lightning flash, the initial stroke does not produce as loud or as long a thunder as subsequent return strokes.
The sounds produced by thunder have been categorized into recognizable terms. Claps are sudden loud sounds lasting 0.2 to 2 seconds. Peals are sounds changing frequency or amplitude. Rolls are irregular sound variances. Rumbles are of long duration but relatively low in frequency. Close-in lightning has been described first as a clicking or cloth-tearing sound, then a cannon shot sound or loud crack/snap, followed by continuous rumbling.
Malan (1963) described these in more technical terms: the click is the upward streamer(s) and the crack is the rumble originating from the upper regions of the channel. A typical thunder episode consists of a rumble and a roll, on which three to four peals or claps are superimposed.
The step leader comes from the cloud toward the ground. Then the return stroke is later. Of course, on the ground, we hear the closet part first, which are the upward streamers, then the step leader, which is farther away but occurred first. People who fear the sound of thunder suffer from a phobia called brontophobia and the fear of lightning is called keraunophobia.
Science of Thunder
Lightning has a diameter of 1-2 inches (2-5 cm) and can heat air to 70,000° F (39,000° C) in a few milliseconds. Ninety percent of the electrical energy of lightning is released in the form of heat, which is quickly dissipated into the atmosphere. Less than 1% of lightning's energy is converted into sound and the rest released in the form of light. A sudden increase in pressure and temperature causes surrounding air to expand violently at a rate faster than the speed of sound, similar to a sonic boom. The shock wave extends outward for the first 30 feet (10 m), after which it becomes an ordinary sound wave called thunder. The speed of sound through air at sea level is 758 mph (1,130 feet/second; 344 m/second) at 68° F (20° C). Thunder is exploding air occurring along the entire length of the lightning channel. An average thunderstorm produces thousands of mi/km of lightning channel during its lifetime.
Sound velocity is proportional to the square root of temperature. Temperature typically decreases with height, unless there is an inversion (warm air over cooler air). Thus, the sound of thunder will be deflected upward. Humidity, wind velocity, wind shear, temperature inversions, terrain features, and clouds, also influence thunder's audibility. The loudness of thunder can be expressed in decibels (dB). A clap of thunder typically registers at about 120 dB in close proximity to the ground stroke. This is 10 times louder than a garbage truck or pneumatic jackhammer drill. By comparison, sitting in front of speakers at a rock concert can expose you to a continuous 120+ dB level. Thunder in close proximity is capable of producing temporary deafness and may cause rupturing of the ear's tympanic membrane that can lead to hearing damage or deafness.
At very close range, thunder is capable of causing property damage. The shock wave, pressure, and propagation of thunder may cause exterior and interior damage to structures. Popping of nail-supported drywall away from horizontal and vertical wooden studs inside houses has been documented. Glass windows have been broken by the concussion of thunder.
Thunder contains a somewhat cylindrical initial pressure shock wave along the lightning channel in excess of 10 times the normal atmospheric pressure. This shock wave decays rapidly into a sound wave within feet or meters. When thunder is heard from about 328 feet (100 m) distance, it consists of one large bang, yet hissing and clicking may be heard just prior to the bang (upward streamers). When heard at .6 mile (1 km) from lightning, thunder will rumble with several loud claps.
Thunder is seldom heard beyond 10 miles (16 km) under ideal conditions. The sound of distant thunder has a characteristic low-pitched rumbling sound. Pitch, the degree of highness or lowness of a sound, is due to strong absorption and scattering of high-frequency components of the original sound waves, while the rumbling results from the fact that sound waves are emitted from different locations along the lightning channel, which lie at varying distances from a person. The longer the lightning channels, the longer the sound of thunder. Humans hear frequencies of thunder between 20-120 Hertz (Hz). However, there is a small amount, less than 10%, that is inaudible to humans produced from lightning, called infrasonic. Special listening devices are required to record these inaudible sounds.
Thunder and Lightning Facts
Lightning is the number two thunderstorm-related killer in the U.S. On average, it kills more people each year than do tornadoes and hurricanes. A hundred years ago, lightning probably was the leading thunderstorm-related killer. At that time, the U.S. economy was predominantly agricultural and labor intensive. The majority of people worked outside, exposing them frequently to the threat of lightning compared to today. In addition, housing was much less substantial, lacking the plumbing and wiring that we have today that acts more or less like a Faraday cage to channel lightning around and away from the inhabitants. It was not unusual for a structure to literally blow apart when hit by lightning, often crushing the inhabitants.
Lightning and its subsequent thunder can be used in lightning safety to protect yourself and others. The flash-to-bang method of protection considers the time between seeing lightning to hearing its thunder. Light from lightning travels at the speed of 186,000 miles per second (300,000 km/second), arriving at the observer in about 10 microseconds when the strike point is 1.85 miles (3 km) away. The sound wave, at an air temperature of 68° F (20° C) and atmospheric pressure of 29.92 in of mercury or 1,013.25 millibars, arrives more slowly in about 10 seconds. Figure 1 shows how a time interval from flash-to-bang of 5 seconds = 1 mile (1.6 km) can be approximated.
The Lightning Safety Group (LSG), an interdisciplinary group of the nation's lightning experts, met at the 1998 American Meteorological Society Annual Meeting. New lightning data showed that most CG flashes in a storm were within 5-6 miles (8-9.6 km) of the previous flash. The LSG recommended what has become known as the 30/30 Rule. Using the flash-to-bang method, lightning that has a 30-second count between the flash and the thunder is 6 miles (9.6 km) away. This translates into 5 seconds per mile (1.6 km). It is possible that the next flash of CG lightning may occur at your location.
The LSG also suggests waiting 30 minutes after hearing the last sound of thunder or seeing the last lightning in daytime before returning to any outside activity. This allows the thunderstorm to move out of the area, greatly reducing the lightning threat level. The average lightning flash distance between two flashes averages about 2-3 mikes (3-5 km), but 6 miles (9.6 km) accounts for about 80% of subsequent CGs. The LSG strongly recommends proactive action rather than a reactive approach to lightning safety. This means knowing the weather prediction and pre-planning for a scene evacuation that includes knowing a safer place to go and the time needed to reach it. Statistics have shown the majority of people who are struck by lightning are struck before or after a thunderstorm, not during the heaviest rain.
A few short rhymes or slogans to remember for lightning safety are as follows:
A lightning flash has brightness, infrared and ultraviolet radiation that can temporarily blind a person or seriously damage vision. Lightning injury and death predominantly occurs outside, often during recreational activities. Lightning injuries often last a lifetime.
In 2000, the National Weather Service, in association with corporate and private sponsors, established Lightning Safety Awareness Week (LSAW). This annual event takes place during the last full week in June. The goal of the LSAW is to reduce injury and death from lightning by promoting awareness and education. Medical information, lightning safety, and support groups for strike victims can be obtained from the following web sites, in addition to this one: www.lightningsafety.noaa.gov/, www.uic.edu/labs/lightninginjury, www.struckbylightning.org, and www.lightning-strike.org. Additional weather and lightning information with related topics may be downloaded from the following web site: www.nssl.noaa.gov/resources.
During the later half of the 19th century, H.F. Kretzer collected thunderstorm newspaper articles. Terminology used to describe lightning and thunder was different than that used today. Instead of using the word thunder, it was described as an unusual acoustical or deadening report, or an acoustical bombardment. Lightning was described as an electrical bombardment or accompaniment, or an electrical pyrotechnic or peculiar pyrotechnic display.
During an 11-hour period on July 17-18, 2003, in a 15-mile (24-km) radius centered in Merrillville, Indiana, there were 10,428 CG flashes. Since the majority (between 50%-90%) of all lightning flashes are IC flashes that do not strike the ground, taking a rate of 10 cloud flashes per CG flashes, this storm had an estimated 104,280 flashes, which corresponds to 158 flashes per min or 2.6 flashes per sec.
For centuries there have been documented records from reliable individuals reporting unusual behavior (anxiety, restlessness, and irritability) associated with some pets and livestock prior to thunderstorm activity. This behavior has been observed in animals for as much as an hour or more before the first sound of thunder is heard in the distance. It is speculated that some animals are reacting to hearing long-wave sound energy below the 20-Hz level from an approaching thunderstorm.
Theories about the cause of thunder date back thousands of years. It wasn't until the turn of the 20th century that the origin of thunder was properly identified and accepted. Thunder is produced by the explosive expansion of heated air surrounding a lightning channel. Thunder can be heard from a maximum distance of about 10 miles (16 km) under good atmospheric conditions. When lightning strikes close, thunder has a loud clap or snapping sound. The rumbling we hear is the sound of thunder reaching us at different times from the sound produced along its length. People have experienced injury and property damage from the sound of thunder at close range.
If outdoor activity is planned for the day, check your local weather forecast for the possibility of thunderstorms. Become proactive about safety, instead of reactive. The sound of thunder can be a wake up call for lightning safety. Practice the 30/30 Rule and visit the lightning safety web sites for additional information. If you see lightning and hear thunder in 30 seconds or less, the threat is imminent and the next strike may be at your location. Take safety measures immediately. Outdoor activities should not be resumed until 30 minutes after the last thunder is heard or lightning seen. Many individuals struck by lightning are struck before or after the heaviest rain from a thunderstorm, not during the strongest part. It appears that people pay more attention to the rain than to the danger from lightning injury.
We greatly appreciate the following individuals for reviewing and improving
this paper by giving their time and knowledge:
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